The heart requires an electrical impulse or signal in order for it to contract and pump blood throughout the body. The electrical impulse of the conduction system begins in the sinoatrial node (the heart's natural pacemaker). The sinoatrial node initiates an electrical impulse that flows through the right and left atria, making them contract. When the electrical impulse reaches the atrioventricular node, it is delayed slightly. The impulse then travels down the Bundle of His, which divides into the right bundle branch for the right ventricle and the left bundle branch for the left ventricle. The impulse then spreads through the ventricles, making them contract.
One heart beat or cardiac cycle refers to the alternating contraction and relaxation of the atria and ventricles. The two atria contract at the same time, then relax, followed by the two ventricles contracting, then relaxing. The contraction phase is called systole. The relaxation phase is called diastole. A well-functioning conduction system is necessary for the heart to contract and pump an adequate supply of blood throughout the body.